Idles high

I just read your question and I wish I had the answer because I have the exact same set of codes as you do and I have started down the same repair road that you are on. I have had this problem in front of four different mechanics and have spent hundreds of dollars in diagnostics. I am tearing my hair out. All the codes point to the TPS
P0507. Idle Control System RPM Higher Than Expected.
P1125. Throttle position motor fail safe mode.
P1275. Accelerator pedal position sensor 1 circuit fault.
P1280. Accelerator pedal position sensor 2 circuit fault.
P1290. Throttle position motor forced idle mode.
P1295. Throttle position motor power management mode.
P1515?
I think there is a wiring problem so I am checking for wiring issues: broken wires, bad grounds, etc. I found a ground strap under a broken bolt on the intake manifold so obviously it looked like it was down tight but of course it was not. I moved it to a secure ground point and the problems temporarily seemed to resolve but then the same codes are back again.

If we put our heads together maybe we can figure this demon out. Keep in touch. Rich

Here is the update on my 2000 Honda Passport (Isuzu) check engine/reduced power TPS problem.

After replacing the TPS (throttle position sensor) with a new one and getting the same codes as before I assumed that I must have a wiring problem. It stands to reason that if the computer is sending out the same error codes before and after part replacement then there must be an information block. I started with the grounding issues because good grounding is essential. Earlier I found a broken bolt in the intake manifold that held a ground. It obviously looked secure but was not. I moved the ground to a secure bolt and for a short while the problem did resolve. Later the problem came back. This again seemed to point me in the direction of a wiring problem because there was an intermittent issuing going on. I cleaned and checked every other ground wire, added new grounding from the battery to the engine, and replaced the battery cables but that did not help this time. I moved next to the computer. I wanted to check that every wire was secure in the socket and that it wasn't broken. Because there are so many wires it is a daunting task. I removed both the blue and red sockets running to the computer. I pulled both directions on every wire on the blue socket and all the wires were secure. I moved next to the red socket.

When I was about half way through that group of wires I pulled on one that seemed much less tight. I pulled a little harder and it slid right out of the harness. Eureka! Now I had to find the other end of the break. I could by examining the end of the broken wire that it was shorted and as I dug through the bundle I found the other end. It was fused to another wire which was also partially if not completely shorted as well and the two had melted together somewhat. Fortunately I was able to identify which wire went where because both wires were color coded the same. I soldered the wires back together, reconnected the plugs to the computer and started the car. The reduced power light immediately went off. The engine accelerated for the first time in weeks and after a few starts the check engine light cleared and also turned off.

My theory is that the bad ground allowed an imbalance of current in the circuit and the guilty wire overheated right in the middle of the harness bundle where the most heat would be prone to be concentrated and where the bundle was packed tightest together perhaps weakening the insulation protection and eventually a direct short resulted.

I hope this helps someone else with a similar problem because I had this car in two very competent shops and neither one of them could figure out the problem. I spent hundreds of dollars in diagnosis but in the end they told me I needed to take it to an Isuzu dealer who had the tailored diagnostic equipment. The closest dealer to my town is one hundred and fifty miles away so I either had to trailer the car there or fix it myself, which is what I did.

I say the mechanics relied too much on the data coming from the computer and missing the possibility that the problem was not a component related failure but a wiring failure. Good luck to the rest of you.